Heat the oven to 180C/350F/gas mark 4. Put the celery in a large nonstick sauté pan with the bay leaves and shaved lemon. Pour over the milk, put the pan on a medium-high heat, bring to a gentle simmer and cook for 12 minutes, until soft.
Strain the celery, reserving 300ml of the milk for later; discard the rest, as well as the bay and lemon.
Wipe clean the pan, fill it with enough water to cover the chard (you'll need about a litre), bring to a boil and add the chard stalks. Boil for two minutes, add the chard leaves and cook for another minute. Drain and pat dry.
Wipe the pan dry, add the butter and put on a medium-high heat. Add the chard, three-quarters of a teaspoon of salt and some black pepper, and sauté for four minutes.
Add the strained celery, cook, stirring, for a minute, then sprinkle in the flour and stir until it cooks a little and absorbs some of the liquid in the pan; this will take about a minute.
Pour the reserved milk over the vegetables, stir for two minutes un-til it starts to thicken, then add the lemon juice. Tip into a 20cm x 20cm gratin dish and set aside.
Mix the breadcrumbs in a bowl with the parsley, parmesan, grated lemon zest and a quarter-teaspoon of salt. Sprinkle this evenly over the gratin, then bake for 30 minutes, until the topping is golden-brown and the sauce bubbling away underneath. Leave to rest for 10 minutes before serving.
